It has recently been discovered that a high flux of illumination by 405 nm radiation could have a significant desirable bactericidal effect. Following this, some companies have proposed creating a white light emitter having a large violet peak, which could provide general-illumination light at the same time as a bactericidal effect. The conventional approach is to combine standard white light emitting diodes (LED)s with violet LEDs emitting at 405 nm.
Applicants recognize, however, that this approach leads to light having poor chromaticity. Chromaticity may be quantified through use the Cartesian distance in (u v) space, known as Duv, or in (x y) space, known as Dxy. For a target chromaticity (for instance, that of a Blackbody radiator at 3000 K), Duv is calculated as the Cartesian distance between that target chromaticity and the light source's actual chromaticity in (u, v) space. In some cases, the distance is computed between a point and a curve—for instance, between the chromaticity of a spectrum (a point) and the Planckian locus (a curve). The distance is the closest distance from the point to the curve (i.e. the distance from the point to its orthogonal projection on the curve, in the space of interest). This concept is commonly used in color science to express how closely an SPD replicates the chromaticity of a blackbody radiator. Color distances may be expressed in values of Duv or Dxy. As known, Duv and Dxy are related. Typically, a Duv value is about half of the corresponding Dxy value (with some variation depending on the specific direction of the color shift). In particular, for shifts substantially along the +/−y direction (as is the case in some embodiments shown herein), this ratio is 0.5. This conversion factor may be used to translate from one distance metric to the other.
FIGS. 1a and 1b illustrate the poor chromaticity of the conventional approach of combining standard white light emitting diodes (LED)s with violet LEDs emitting at 405 nm. In FIG. 1a, light emitted from a standard white LED (white spectrum 102, on-Planckian with correlated color temperature (CCT)=3000 K, Ra=80, R9˜0) is combined with a large violet peak from a violet LED (violet spectrum 104, having a peak at 405 nm). The resulting spectrum 106 (has a high violet content, and may be suitable for bactericidal purposes. However, the addition of the violet peak pulls the chromaticity to a higher CCT (3200K) and far below-Planckian (Duv=−0.0177), resulting in an uncontrolled very pronounced pink tint.
Furthermore, depending on the amount of violet light, the resulting chromaticity may occur at any uncontrolled color point, which can be undesirable in applications where a controlled chromaticity (often, substantially on-Planckian) is wanted. This is further illustrated in FIG. 1b. FIG. 1b shows the chromaticity (in x-y diagram) of the same standard white LED, when combined with various amounts of violet light. Curve 108 is the Planckian locus. The percent values shown on FIG. 1b correspond to the violet fraction, i.e. the fraction of the spectral power distribution (SPD) in the range 390-420 nm. At 0% violet, the source is on-Planckian. In other words, a standard white LED comprising 0% violet is on-Planckian. As the violet fraction increases to 10%, 20%, and 30%, however, the chromaticity is pulled below-Planckian, and the emitted light is no longer white. This is characterized by the Duv distance from the Planckian locus—i.e., −0.0044, −0.0102 and −0.0177, respectively.
The large values of Duv distance from Planckian demonstrate that it is not possible to add much violet light and remain near the Planckian locus with this approach, while emitting white light. The last point of FIG. 1b has 30% violet, and corresponds to the SPD of composite spectrum 106 of FIG. 1a. Therefore, there is a need for a light source with a spectrum having a large fraction of violet light but retaining desirable quality of white light. The present invention fulfills this need among others.